The combustion of solid waste in a Municipal Waste Combustor (MWC) generates some amount of NOx. NOx is the generic name for a group of colorless and odorless but highly reactive gases that contain varying amounts of NO and NO2. The amount of NOx generated by the MWCs varies somewhat according to the grate and furnace design but typically ranges between 250 and 350 ppm (dry value at 7% O2 in the flue gas).
The chemistry of NOx formation is directly tied to reactions between nitrogen and oxygen. To understand NOx formation in a MWC, a basic understanding of combustor design and operation is useful. Combustion air systems in MWCs typically include both primary (also called undergrate) air and secondary (also called overgrate or overfire) air. Primary air is supplied through plenums located under the firing grate and is forced through the grate to sequentially dry (evolve water), devolatilize (evolve volatile hydrocarbons), and burn out (oxidize nonvolatile hydrocarbons) the waste bed. The quantity of primary air is typically adjusted to minimize excess air during initial combustion of the waste while maximizing burnout of carbonaceous materials in the waste bed. Secondary air is injected through airports located above the grate and is used to provide turbulent mixing and destruction of hydrocarbons evolved from the waste bed. Overall excess air levels for a typical MWC are approximately 60 to 100% (160-200% of stoichiometric (i.e., theoretical) air requirements), with primary air typically accounting for 50-70% of the total air.
In addition to destruction of organics, one of the objectives of this combustion approach is to minimize NOx formation. NOx is formed during combustion through two primary mechanisms: Fuel NOx from oxidation of organically bound elemental nitrogen (N) present in the municipal solid waste (MSW) stream and Thermal NOx from high temperature oxidation of atmospheric N2.
More specifically, fuel NOx is formed within the flame zone through reaction of organically bound N in MSW materials and O2. Key variables determining the rate of fuel NOx formation are the availability of O2 within the flame zone, the amount of fuel-bound N, and the chemical structure of the N-containing material. Fuel NOx reactions can occur at relatively low temperatures (<1,100° C. (<2,000° F.)). Depending on the availability of O2 in the flame, the N-containing compounds will react to form either N2 or NOx. When the availability of O2 is low, N2 is the predominant reaction product. If substantial O2 is available, an increased fraction of the fuel-bound N is converted to NOx.
In contrast, thermal NOx is formed in high-temperature flame zones through reactions between N2 and O2 radicals. The key variables determining the rate of thermal NOx formation are temperature, the availability of O2 and N2, and residence time. Because of the high activation energy required, thermal NOx formation does not become significant until flame temperatures reach 1,100° C. (2,000° F.).
However, NOx emissions are generally undesirable and are of environmental significance because of their role as a criteria pollutant, acid gas, and ozone precursor. Direct health concerns of NOx center on the gases' effects on the respiratory system because NOx reacts with ammonia, moisture and other compounds to form nitric acid and related particles that may damage lung tissue. These and other particles produced from NOx penetrate deeply into sensitive parts of the lungs and can cause or worsen potentially fatal respiratory diseases such as emphysema and bronchitis.
In addition, the emissions of NOx pose other environmental concerns. For example, ground-level ozone is formed when NOx and volatile organic compounds (VOCs) react with heat and sunlight. Children, asthmatics, and people who work or exercise outside are susceptible to adverse effects from the ozone, and these effects include lung tissue damage and decreased lung function. Ozone also damages vegetation and reduces crop yields.
Furthermore, the reaction of NOx and sulfur dioxide with other substances in the air to form acids, which fall to earth with rain, fog, snow or dry particles as acid rain. Acid rain damages or deteriorates cars, buildings and monuments, as well as causes lakes and streams to become unsuitable for fish.
In addition, NOx are indirect greenhouse gases that affect the atmospheric amounts of hydroxyl (OH) radicals. Specifically, the breakdown of NOx gases gives rise to increased OH abundance.
Consequently, various laws and regulations have been passed to limit the emissions of NOx from MWCs and other sources. For example, the Unites States Environmental Agency is authorized in 40 C.F.R. Part 60 to monitor and limit NOx from MWCs. Similar rules and regulations to limit NOx emissions likewise exist internationally, such as in Europe, Canada, and Japan. It should be appreciated that a complete understanding and knowledge of various rules and laws on NOx emissions are outside the scope of the current discussion.
NOx control technologies can be divided into two subgroups: combustion controls and post-combustion controls. Combustion controls limit the formation of NOx during the combustion process by reducing the availability of O2 within the flame and lowering combustion zone temperatures. These technologies include staged combustion, low excess air, and flue gas recirculation (FGR). Staged combustion and low excess air reduce the flow of undergrate air in order to reduce O2 availability in the combustion zone, which promotes chemical reduction of some of the NOx formed during primary combustion. In FGR, a portion of the combustor exhaust is returned to the combustion air supply to both lower combustion zone O2 and suppress flame temperatures by reducing the ratio of O2 to inerts (N2 and carbon dioxide (CO2)) in the combustion air system.
Post-combustion controls relate to removing NOx emissions produced during the combustion process at solid waste fired boilers, and the most commonly used post-combustion NOx controls include selective non-catalytic reduction (SNCR) systems, which typically reduce the NOx significantly, or selective catalytic reduction (SCR) systems, which typically reduce the NOx even more effectively than SNCR systems. As described in greater detail below, SCR systems are many times more expensive to build, operate, and maintain than SNCR systems and are consequently not economically feasible for use on waste-to-energy (WTE) plants in many parts of the world.
SCR is an add-on control technology that catalytically promotes the reaction between NH3 and NOx. SCR systems can use aqueous or anhydrous NH3 reagent, with the primary differences being the size of the NH3 vaporization system and the safety requirements. In the SCR system, a precise amount of a reagent is metered into the exhaust stream. The reagent decomposes into ammonia and reacts with NOx across a catalyst located downstream of the injection point. This reaction reduces NOx to elemental nitrogen and water vapor. SCR systems typically operate at temperature of approximately 500-700° F. In terms of waste disposal fee impact and cost effectiveness, SCR generally has higher costs resulting from high capital costs, as well as the cost of catalyst replacement and disposal.
In contrast, SNCR reduces NOx to N2 without the use of catalysts. Similar to the SCR system, the SNCR system injects one or more reducing agents into the upper furnace of the MWC to react with NOx and form N2. Without the assistance of a catalyst, these reactions occur at temperatures of approximately 1600-1800° F. Operation of SNCR processes near the upper end of their performance range may result in unwanted emissions of ammonia or other by-product gases. SNCR generally has significantly lower capital costs, as well as lower maintenance costs since there are no catalysts to replace and dispose.